System and method for reducing command scheduling constraints of memory circuits

ABSTRACT

A memory circuit system and method are provided. An interface circuit is capable of communication with a plurality of memory circuits and a system. In use, the interface circuit is operable to interface the memory circuits and the system for reducing command scheduling constraints of the memory circuits.

RELATED APPLICATION(S)

This application is a continuation-in-part of U.S. application Ser. No.: 11/461,437 filed Jul. 31, 2006, which is now U.S. Pat. No. 8,077,535, U.S. application Ser. No. 11/702,981 filed Feb. 05, 2007, which is now U.S. Pat. No. 8,089,795, and U.S. application Ser. No. 11/702,960 filed Feb. 05, 2007 : and further claims the benefit of U.S. provisional application Ser. No.: 60/772,414 filed Feb. 9, 2006 and U.S. provisional application Ser. No.: 60/865,624 filed Nov. 13, 2006, which are each incorporated herein by reference in their entirety for all purposes. The present application is also related to U.S. application Ser. No. 11/672,924 filed Feb. 8, 2007 which names identical inventorship and is incorporated herein by reference in its entirety for all purposes.

FIELD OF THE INVENTION

The present invention relates to memory, and more particularly to command scheduling constraints of memory circuits.

BACKGROUND

Traditionally, memory circuit speeds have remained relatively constant, while the required data transfer speeds and bandwidth of memory systems have steadily increased. Thus, it has been necessary for more commands be scheduled, issued, and pipelined in a memory system in order to increase bandwidth. However, command scheduling constraints have customarily existed in memory systems which limit the command issue rates, and thus limit various attempts to further increase bandwidth, etc. There is thus a need for addressing these and/or other issues associated with the prior art.

SUMMARY

A memory circuit system and method are provided. An interface circuit is capable of communication with a plurality of memory circuits and a system. In use, the interface circuit is operable to interface the memory circuits and the system for reducing command scheduling constraints of the memory circuits.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a system for interfacing memory circuits, in accordance with one embodiment.

FIG. 2 illustrates a method for reducing command scheduling constraints of memory circuits, in accordance with another embodiment.

FIG. 3 illustrates a method for translating an address associated with a command communicated between a system and memory circuits, in accordance with yet another embodiment.

FIG. 4 illustrates a block diagram including logical components of a computer platform, in accordance with another embodiment.

FIG. 5 illustrates a timing diagram showing an intra-device command sequence, intra-device timing constraints, and resulting idle cycles that prevent full use of bandwidth utilization in a DDR3 SDRAM memory system, in accordance with yet another embodiment.

FIG. 6 illustrates a timing diagram showing an inter-device command sequence, inter-device timing constraints, and resulting idle cycles that prevent full use of bandwidth utilization in a DDR SDRAM, DDR2 SDRAM, or DDR3 SDRAM memory system, in accordance with still yet another embodiment.

FIG. 7 illustrates a block diagram showing an array of DRAM devices connected to a memory controller, in accordance with another embodiment.

FIG. 8 illustrates a block diagram showing an interface circuit disposed between an array of DRAM devices and a memory controller, in accordance with yet another embodiment.

FIG. 9 illustrates a block diagram showing a DDR3 SDRAM interface circuit disposed between an array of DRAM devices and a memory controller, in accordance with another embodiment.

FIG. 10 illustrates a block diagram showing a burst-merging interface circuit connected to multiple DRAM devices with multiple independent data buses, in accordance with still yet another embodiment.

FIG. 11 illustrates a timing diagram showing continuous data transfer over multiple commands in a command sequence, in accordance with another embodiment.

FIG. 12 illustrates a block diagram showing a protocol translation and interface circuit connected to multiple DRAM devices with multiple independent data buses, in accordance with yet another embodiment.

FIG. 13 illustrates a timing diagram showing the effect when a memory controller issues a column-access command late, in accordance with another embodiment.

FIG. 14 illustrates a timing diagram showing the effect when a memory controller issues a column-access command early, in accordance with still yet another embodiment.

FIG. 15 illustrates a representative hardware environment, in accordance with one embodiment.

DETAILED DESCRIPTION

FIG. 1 illustrates a system 100 for interfacing memory circuits, in accordance with one embodiment. As shown, the system 100 includes an interface circuit 104 in communication with a plurality of memory circuits 102 and a system 106. In the context of the present description, such memory circuits 102 may include any circuits capable of serving as memory.

For example, in various embodiments, at least one of the memory circuits 102 may include a monolithic memory circuit, a semiconductor die, a chip, a packaged memory circuit, or any other type of tangible memory circuit. In one embodiment, the memory circuits 102 may take the form of dynamic random access memory (DRAM) circuits. Such DRAM may take any form including, but not limited to, synchronous DRAM (SDRAM), double data rate synchronous DRAM (DDR SDRAM, DDR2 SDRAM, DDR3 SDRAM, etc.), graphics double data rate DRAM (GDDR, GDDR2, GDDR3, etc.), quad data rate DRAM (QDR DRAM), RAMBUS XDR DRAM (XDR DRAM), fast page mode DRAM (FPM DRAM), video DRAM (VDRAM), extended data out DRAM (EDO DRAM), burst EDO RAM (BEDO DRAM), multibank DRAM (MDRAM), synchronous graphics RAM (SGRAM), and/or any other type of DRAM.

In another embodiment, at least one of the memory circuits 102 may include magnetic random access memory (MRAM), intelligent random access memory (IRAM), distributed network architecture (DNA) memory, window random access memory (WRAM), flash memory (e.g. NAND, NOR, etc.) pseutostatic random access memory (PSRAM), wetware memory, memory based on semiconductor, atomic, molecular, optical, organic, biological, chemical, or nanoscale technology, and/or any other type of volatile or nonvolatile, random or non-random access, serial or parallel access memory circuit.

Strictly as an option, the memory circuits 102 may or may not be positioned on at least one dual in-line memory module (DIMM) (not shown). In various embodiments, the DIMM may include a registered DIMM (R-DIMM), a small outline-DIMM (SO-DIMM), a fully buffered DIMM (FB-DIMM), an unbuffered DIMM (UDIMM), single inline memory module (SIMM), a MiniDIMM, a very low profile (VLP) R-DIMM, etc. In other embodiments, the memory circuits 102 may or may not be positioned on any type of material forming a substrate, card, module, sheet, fabric, board, carrier or any other type of solid or flexible entity, form, or object. Of course, in yet other embodiments, the memory circuits 102 may or may not be positioned in or on any desired entity, form, or object for packaging purposes. Still yet, the memory circuits 102 may or may not be organized into ranks. Such ranks may refer to any arrangement of such memory circuits 102 on any of the foregoing entities, forms, objects, etc.

Further, in the context of the present description, the system 106 may include any system capable of requesting and/or initiating a process that results in an access of the memory circuits 102. As an option, the system 106 may accomplish this utilizing a memory controller (not shown), or any other desired mechanism. In one embodiment, such system 106 may include a system in the form of a desktop computer, a lap-top computer, a server, a storage system, a networking system, a workstation, a personal digital assistant (PDA), a mobile phone, a television, a computer peripheral (e.g. printer, etc.), a consumer electronics system, a communication system, and/or any other software and/or hardware, for that matter.

The interface circuit 104 may, in the context of the present description, refer to any circuit capable of interfacing (e.g. communicating, buffering, etc.) with the memory circuits 102 and the system 106. For example, the interface circuit 104 may, in the context of different embodiments, include a circuit capable of directly (e.g. via wire, bus, connector, and/or any other direct communication medium, etc.) and/or indirectly (e.g. via wireless, optical, capacitive, electric field, magnetic field, electromagnetic field, and/or any other indirect communication medium, etc.) communicating with the memory circuits 102 and the system 106. In additional different embodiments, the communication may use a direct connection (e.g. point-to-point, single-drop bus, multi-drop bus, serial bus, parallel bus, link, and/or any other direct connection, etc.) or may use an indirect connection (e.g. through intermediate circuits, intermediate logic, an intermediate bus or busses, and/or any other indirect connection, etc.).

In additional optional embodiments, the interface circuit 104 may include one or more circuits, such as a buffer (e.g. buffer chip, etc.), a register (e.g. register chip, etc.), an advanced memory buffer (AMB) (e.g. AMB chip, etc.), a component positioned on at least one DIMM, a memory controller, etc. Moreover, the register may, in various embodiments, include a JEDEC Solid State Technology Association (known as JEDEC) standard register (a JEDEC register), a register with forwarding, storing, and/or buffering capabilities, etc. In various embodiments, the register chips, buffer chips, and/or any other interface circuit 104 may be intelligent, that is, include logic that is capable of one or more functions such as gathering and/or storing information, inferring, predicting, and/or storing state and/or status; performing logical decisions; and/or performing operations on input signals, etc. In still other embodiments, the interface circuit 104 may optionally be manufactured in monolithic form, packaged form, printed form, and/or any other manufactured form of circuit, for that matter. Furthermore, in another embodiment, the interface circuit 104 may be positioned on a DIMM.

In still yet another embodiment, a plurality of the aforementioned interface circuit 104 may serve, in combination, to interface the memory circuits 102 and the system 106. Thus, in various embodiments, one, two, three, four, or more interface circuits 104 may be utilized for such interfacing purposes. In addition, multiple interface circuits 104 may be relatively configured or connected in any desired manner. For example, the interface circuits 104 may be configured or connected in parallel, serially, or in various combinations thereof. The multiple interface circuits 104 may use direct connections to each other, indirect connections to each other, or even a combination thereof. Furthermore, any number of the interface circuits 104 may be allocated to any number of the memory circuits 102. In various other embodiments, each of the plurality of interface circuits 104 may be the same or different. Even still, the interface circuits 104 may share the same or similar interface tasks and/or perform different interface tasks.

While the memory circuits 102, interface circuit 104, and system 106 are shown to be separate parts, it is contemplated that any of such parts (or portion(s) thereof) may be integrated in any desired manner. In various embodiments, such optional integration may involve simply packaging such parts together (e.g. stacking the parts to form a stack of DRAM circuits, a DRAM stack, a plurality of DRAM stacks, a hardware stack, where a stack may refer to any bundle, collection, or grouping of parts and/or circuits, etc.) and/or integrating them monolithically. Just by way of example, in one optional embodiment, at least one interface circuit 104 (or portion(s) thereof) may be packaged with at least one of the memory circuits 102. In this way, the interface circuit 104 and the memory circuits 102 may take the form of a stack, in one embodiment.

For example, a DRAM stack may or may not include at least one interface circuit 104 (or portion(s) thereof). In other embodiments, different numbers of the interface circuit 104 (or portion(s) thereof) may be packaged together. Such different packaging arrangements, when employed, may optionally improve the utilization of a monolithic silicon implementation, for example.

The interface circuit 104 may be capable of various functionality, in the context of different optional embodiments. Just by way of example, the interface circuit 104 may or may not be operable to interface a first number of memory circuits 102 and the system 106 for simulating a second number of memory circuits 105 to the system 106. In the illustrated figure, the second number of memory circuits 105 is shown with a dashed border to indicate that its memory circuits are simulated. The first number of memory circuits 102 shall hereafter be referred to, where appropriate for clarification purposes, as the “physical” memory circuits 102 or memory circuits, but are not limited to be so. Just by way of example, the physical memory circuits 102 may include a single physical memory circuit. Further, the at least one simulated memory circuit seen by the system 106 shall hereafter be referred to, where appropriate for clarification purposes, as the at least one “virtual” memory circuit.

In still additional aspects of the present embodiment, the second number of virtual memory circuits may be more than, equal to, or less than the first number of physical memory circuits 102. Just by way of example, the second number of virtual memory circuits may include a single memory circuit. Of course, however, any number of memory circuits may be simulated.

In the context of the present description, the term simulated may refer to any simulating, emulating, disguising, transforming, modifying, changing, altering, shaping, converting, etc., which results in at least one aspect of the memory circuits 102 appearing different to the system 106. In different embodiments, such aspect may include, for example, a number, a signal, a memory capacity, a timing, a latency, a design parameter, a logical interface, a control system, a property, a behavior, and/or any other aspect, for that matter.

In different embodiments, the simulation may be electrical in nature, logical in nature, protocol in nature, and/or performed in any other desired manner. For instance, in the context of electrical simulation, a number of pins, wires, signals, etc. may be simulated. In the context of logical simulation, a particular function or behavior may be simulated. In the context of protocol, a particular protocol (e.g. DDR3, etc.) may be simulated. Further, in the context of protocol, the simulation may effect conversion between different protocols (e.g. DDR2 and DDR3) or may effect conversion between different versions of the same protocol (e.g. conversion of 4-4-4 DDR2 to 6-6-6 DDR2).

More illustrative information will now be set forth regarding various optional architectures and uses in which the foregoing system may or may not be implemented, per the desires of the user. It should be strongly noted that the following information is set forth for illustrative purposes and should not be construed as limiting in any manner. Any of the following features may be optionally incorporated with or without the exclusion of other features described.

FIG. 2 illustrates a method 200 for reducing command scheduling constraints of memory circuits, in accordance with another embodiment. As an option, the method 200 may be implemented in the context of the system 100 of FIG. 1. Of course, however, the method 200 may be implemented in any desired environment. Further, the aforementioned definitions may equally apply to the description below.

As shown in operation 202, a plurality of memory circuits and a system are interfaced. In one embodiment, the memory circuits and system may be interfaced utilizing an interface circuit. The interface circuit may include, for example, the interface circuit described above with respect to FIG. 1. In addition, in one embodiment, the interfacing may include facilitating communication between the memory circuits and the system. Of course, however, the memory circuits and system may be interfaced in any desired manner.

Further, command scheduling constraints of the memory circuits are reduced, as shown in operation 204. In the context of the present description, the command scheduling constraints include any limitations associated with scheduling (and/or issuing) commands with respect to the memory circuits. Optionally, the command scheduling constraints may be defined by manufacturers in their memory device data sheets, by standards organizations such as the JEDEC, etc.

In one embodiment, the command scheduling constraints may include intra-device command scheduling constraints. Such intra-device command scheduling constraints may include scheduling constraints within a device. For example, the intra-device command scheduling constraints may include a column-to-column delay time (tCCD), row-to-row activation delay time (tRRD), four-bank activation window time (tFAW), write-to-read turn-around time (tWTR), etc. As an option, the intra-device command-scheduling constraints may be associated with parts (e.g. column, row, bank, etc.) of a device (e.g. memory circuit) that share a resource within the memory circuit. One example of such intra-device command scheduling constraints will be described in more detail below with respect to FIG. 5 during the description of a different embodiment.

In another embodiment, the command scheduling constraints may include inter-device command scheduling constraints. Such inter-device scheduling constraints may include scheduling constraints between memory circuits. Just by way of example, the inter-device command scheduling constraints may include rank-to-rank data bus turnaround times, on-die-termination (ODT) control switching times, etc. Optionally, the inter-device command scheduling constraints may be associated with memory circuits that share a resource (e.g. a data bus, etc.) which provides a connection therebetween (e.g. for communicating, etc.). One example of such inter-device command scheduling constraints will be described in more detail below with respect to FIG. 6 during the description of a different embodiment.

Further, reduction of the command scheduling restraints may include complete elimination and/or any decrease thereof. Still yet, in one optional embodiment, the command scheduling constraints may be reduced by controlling the manner in which commands are issued to the memory circuits. Such commands may include, for example, row-access commands, column-access commands, etc. Moreover, in additional embodiments, the commands may optionally be issued to the memory circuits utilizing separate buses associated therewith. One example of memory circuits associated with separate buses will be described in more detail below with respect to FIG. 8 during the description of a different embodiment.

In one possible embodiment, the command scheduling constraints may be reduced by issuing commands to the memory circuits based on simulation of a virtual memory circuit. For example, the plurality of physical memory circuits and the system may be interfaced such that the memory circuits appear to the system as a virtual memory circuit. Such simulated virtual memory circuit may optionally include the virtual memory circuit described above with respect to FIG. 1.

In addition, the virtual memory circuit may have less command scheduling constraints than the physical memory circuits. For example, in one exemplary embodiment, the physical memory circuits may appear as a group of one or more virtual memory circuits that are free from command scheduling constraints. Thus, as an option, the command scheduling constraints may be reduced by issuing commands directed to a single virtual memory circuit, to a plurality of different physical memory circuits. In this way, idle data-bus cycles may optionally be eliminated and memory system bandwidth may be increased.

Of course, it should be noted that the command scheduling constraints may be reduced in any desired manner. Accordingly, in one embodiment, the interface circuit may be utilized to eliminate, at least in part, inter-device and/or intra-device command scheduling constraints of memory circuits. Furthermore, reduction of the command scheduling constraints of the memory circuits may result in increased command issue rates. For example, a greater amount of commands may be issued to the memory circuits by reducing limitations associated with the command scheduling constraints. More information regarding increasing command issue rates by reducing command scheduling constraints will be described with respect to FIG. 11 during the description of a different embodiment.

FIG. 3 illustrates a method 300 for translating an address associated with a command communicated between a system and memory circuits, in accordance with yet another embodiment. As an option, the method 300 may be carried out in context of the architecture and environment of FIGS. 1 and/or 2. Of course, the method 300 may be carried out in any desired environment. Further, the aforementioned definitions may equally apply to the description below.

As shown in operation 302, a plurality of memory circuits and a system are interfaced. In one embodiment, the memory circuits and system may be interfaced utilizing an interface circuit, such as that described above with respect to FIG. 1, for example. In one embodiment, the interfacing may include facilitating communication between the memory circuits and the system. Of course, however, the memory cicuits and system may be interfaced in any desired manner.

Additionally, an address associated with a command communicated between the system and the memory circuits is translated, as shown in operation 304. Such command may include, for example, a row-access command, a column-access command, and/or any other command capable of being communicated between the system and the memory circuits. As an option, the translation may be transparent to the system. In this way, the system may issue a command to the memory circuits, and such command may be translated without knowledge and/or input by the system. Of course, embodiments are contemplated where such transparency is non-existent, at least in part.

Further, the address may be translated in any desired manner. In one embodiment, the translation of the address may include shifting the address. In another embodiment, the address may be translated by mapping the address. Optionally, as described above with respect to FIGS. 1 and/or 2, the memory circuits may include physical memory circuits and the interface circuit may simulate at least one virtual memory circuit. To this end, the virtual memory circuit may optionally have a different (e.g. greater, etc.) number of row addresses associated therewith than the physical memory circuits.

Thus, in one possible embodiment, the translation may be performed as a function of the difference in the number of row addresses. For example, the translation may translate the address to reflect the number of row addresses of the virtual memory circuit. In still yet another embodiment, the translation may optionally translate the address as a function of a column address and a row address.

Thus, in one exemplary embodiment where the command includes a row-access command, the translation may be performed as a function of an expected arrival time of a column-access command. In another exemplary embodiment, where the command includes a row-access command, the translation may ensure that a column-access command addresses an open bank. Optionally, the interface circuit may be operable to delay the command communicated between the system and the memory circuits. To this end, the translation may result in sub-row activation of the memory circuits. Various examples of address translation will be described in more detail below with respect to FIG. 8 and 12 during the description of different embodiments.

Accordingly, in one embodiment, address mapping may use shifting of an address from one command to another to allow the use of memory circuits with smaller rows to emulate a larger memory circuit with larger rows. Thus, sub-row activation may be provided. Such sub-row activation may also reduce power consumption and may optionally further improve performance, in various embodiments.

One exemplary embodiment will now be set forth. It should be strongly noted that the following example is set forth for illustrative purposes only and should not be construed as limiting in any manner whatsoever. Specifically, memory storage cells of DRAM devices may be arranged into multiple banks, each bank having multiple rows, and each row having multiple columns. The memory storage capacity of the DRAM device may be equal to the number of banks times the number of rows per bank times the number of column per row times the number of storage bits per column. In commodity DRAM devices (e.g. SDRAM, DDR, DDR2, DDR3, DDR4, GDDR2, GDDR3 and GDDR4 and SDRAM, etc.), the number of banks per device, the number of rows per bank, the number of columns per row, and the column sizes may be determined by a standards-forming committee, such as the Joint Electron Device Engineering Council (JEDEC).

For example, JEDEC standards require that a 1 gigabyte (Gb) DDR2 or DDR3 SDRAM device with a four-bit wide data bus have eight banks per device, 8192 rows per bank, 2048 columns per row, and four bits per column. Similarly, a 2 Gb device with a four-bit wide data bus has eight banks per device, 16384 rows per bank, 2048 columns per row, and four bits per column. A 4 Gb device with a four-bit wide data bus has eight banks per device, 32768 rows per bank, 2048 columns per row, and four bits per column. In the 1 Gb, 2 Gb and 4 Gb devices, the row size is constant, and the number of rows doubles with each doubling of device capacity. Thus, a 2 Gb or a 4 Gb device may be simulated, as described above, by using multiple 1 Gb and 2 Gb devices, and by directly translating row-activation commands to row-activation commands and column-access commands to column-access commands. In one embodiment, this emulation may be possible because the 1 Gb, 2 Gb, and 4 Gb devices have the same row size.

FIG. 4 illustrates a block diagram including logical components of a computer platform 400, in accordance with another embodiment. As an option, the computer platform 400 may be implemented in context of the architecture and environment of FIGS. 1-3. Of course, the computer platform 400 may be implemented in any desired environment. Further, the aforementioned definitions may equally apply to the description below.

As shown, the computer platform 400 includes a system 420. The system 420 includes a memory interface 421, logic for retrieval and storage of external memory attribute expectations 422, memory interaction attributes 423, a data processing engine 424, and various mechanisms to facilitate a user interface 425. The computer platform 400 may be comprised of wholly separate components, namely a system 420 (e.g. a motherboard, etc.), and memory circuits 410 (e.g. physical memory circuits, etc.). In addition, the computer platform 400 may optionally include memory circuits 410 connected directly to the system 420 by way of one or more sockets.

In one embodiment, the memory circuits 410 may be designed to the specifics of various standards, including for example, a standard defining the memory circuits 410 to be JEDEC-compliant semiconductor memory (e.g. DRAM, SDRAM, DDR2, DDR3, etc.). The specifics of such standards may address physical interconnection and logical capabilities of the memory circuits 410.

In another embodiment, the system 420 may include a system BIOS program (not shown) capable of interrogating the physical memory circuits 410 (e.g. DIMMs) to retrieve and store memory attributes 422, 423. Further, various types of external memory circuits 410, including for example JEDEC-compliant DIMMs, may include an EEPROM device known as a serial presence detect (SPD) where the DIMM memory attributes are stored. The interaction of the BIOS with the SPD and the interaction of the BIOS with the memory circuit physical attributes may allow the system memory attribute expectations 422 and memory interaction attributes 423 become known to the system 420.

In various embodiments, the computer platform 400 may include one or more interface circuits 470 electrically disposed between the system 420 and the physical memory circuits 410. The interface circuit 470 may include several system-facing interfaces (e.g. a system address signal interface 471, a system control signal interface 472, a system clock signal interface 473, a system data signal interface 474, etc.). Similarly, the interface circuit 470 may include several memory-facing interfaces (e.g. a memory address signal interface 475, a memory control signal interface 476, a memory clock signal interface 477, a memory data signal interface 478, etc.).

Still yet, the interface circuit 470 may include emulation logic 480. The emulation logic 480 may be operable to receive and optionally store electrical signals (e.g. logic levels, commands, signals, protocol sequences, communications, etc.) from or through the system-facing interfaces, and may further be operable to process such electrical signals. The emulation logic 480 may respond to signals from system-facing interfaces by responding back to the system 420 and presenting signals to the system 420, and may also process the signals with other information previously stored. As another option, the emulation logic 480 may present signals to the physical memory circuits 410. Of course, however, the emulation logic 480 may perform any of the aforementioned functions in any order.

Moreover, the emulation logic 480 may be operable to adopt a personality, where such personality is capable of defining the physical memory circuit attributes. In various embodiments, the personality may be effected via any combination of bonding options, strapping, programmable strapping, the wiring between the interface circuit 470 and the physical memory circuits 410. Further, the personality may be effected via actual physical attributes (e.g. value of mode register, value of extended mode register) of the physical memory circuits 410 connected to the interface circuit 470 as determined when the interface circuit 470 and physical memory circuits 410 are powered up.

FIG. 5 illustrates a timing diagram 500 showing an intra-device command sequence, intra-device timing constraints, and resulting idle cycles that prevent full use of bandwidth utilization in a DDR3 SDRAM memory system, in accordance with yet another embodiment. As an option, the timing diagram 50 may be associated with the architecture and environment of FIGS. 1-4. Of course, the timing diagram 500 may be associated with any desired environment. Further, the aforementioned definitions may equally apply to the description below.

As shown, the timing diagram 500 illustrates command cycles, timing constraints and idle cycles of memory. For example, in an embodiment involving DDR3 SDRAM memory systems, any two row-access commands directed to a single DRAM device may not necessarily be scheduled closer than tRRD. As another example, at most four row-access commands may be scheduled within tFAW to a single DRAM device. Moreover, consecutive column-read access commands and consecutive column-write access commands may not necessarily be scheduled to a given DRAM device any closer than tCCD, where tCCD equals four cycles (eight half-cycles of data) in DDR3 DRAM devices.

In the context of the present embodiment, row-access and/or row-activation commands are shown as ACT. In addition, column-access commands are shown as READ or WRITE. Thus, for example, in memory systems that require a data access in a data burst of four half-cycles, as shown in FIG. 2, the tCCD constraint may prevent column accesses from being scheduled consecutively. Further, the constraints 510, 520 imposed on the DRAM commands sent to a given DRAM device may restrict the command rate, resulting in idle cycles or bubbles 530 on the data bus, therefore reducing the bandwidth.

In another optional embodiment involving DDR3 SDRAM memory systems, consecutive column-access commands sent to different DRAM devices on the same data bus may not necessarily be scheduled any closer than a period that is the sum of the data burst duration plus additional idle cycles due to rank-to-rank data bus turn-around times. In the case of column-read access commands, two DRAM devices on the same data bus may represent two bus masters. Optionally, at least one idle cycle on the bus may be needed for one bus master to complete delivery of data to the memory controller and release control of the shared data bus, such that another bus master may gain control of the data bus and begin to send data.

FIG. 6 illustrates a timing diagram 600 showing inter-device command sequence, inter-device timing constraints, and resulting idle cycles that prevent full use of bandwidth utilization in a DDR SDRAM, DDR2 SDRAM, or DDR3 SDRAM memory system, in accordance with still yet another embodiment. As an option, the timing diagram 600 may be associated with the architecture and environment of FIGS. 1-4. Of course, the timing diagram 600 may be associated with any desired environment. Further, the aforementioned definitions may equally apply to the description below.

As shown, the timing diagram 600 illustrates commands issued to different devices that are free from constraints such as tRRD and tCCD which would otherwise be imposed on commands issue to the same device. However, as also shown, the data bus hand-off from one device to another device requires at least one idle data-bus cycle 610 on the data bus. Thus, the timing diagram 600 illustrates a limitation preventing full use of bandwidth utilization in a DDR3 SDRAM memory system. As a consequence of the command-scheduling constraints, there may be no available command sequence that allows full bandwidth utilization in a DDR3 SDRAM memory system, which also uses bursts shorter than tCCD.

FIG. 7 illustrates a block diagram 700 showing an array of DRAM devices connected to a memory controller, in accordance with another embodiment. As an option, the block diagram 700 may be associated with the architecture and environment of FIGS. 1-6. Of course, the block diagram 700 may be associated with any desired environment. Further, the aforementioned definitions may equally apply to the description below.

As shown, eight DRAM devices are connected directly to a memory controller through a shared data bus 710. Accordingly, commands from the memory controller that are directed to the DRAM devices may be issued with respect to command scheduling constraints (e.g. tRRD, tCCD, tFAW, tWTR, etc.). Thus, the issuance of commands may be delayed based on such command scheduling constraints.

FIG. 8 illustrates a block diagram 800 showing an interface circuit disposed between an array of DRAM devices and a memory controller, in accordance with yet another embodiment. As an option, the block diagram 800 may be associated with the architecture and environment of FIGS. 1-6. Of course, the block diagram 800 may be associated with any desired environment. Further, the aforementioned definitions may equally apply to the description below.

As shown, an interface circuit 810 provides a DRAM interface to the memory controller 820, and directs commands to independent DRAM devices 830. The memory devices 830 may each be associated with a different data bus 540, thus preventing inter-device constraints. In addition, individual and independent memory devices 830 may be used to emulate part of a virtual memory device (e.g. column, row, bank, etc.). Accordingly, intra-device constraints may also be prevented. To this end, the memory devices 830 connected to the interface circuit 510 may appear to the memory controller 820 as a group of one or more memory devices 530 that are free from command-scheduling constraints.

In one exemplary embodiment, N physical DRAM devices may be used to emulate M logical DRAM devices through the use of the interface circuit. The interface circuit may accept a command stream from a memory controller directed toward the M logical devices. The interface circuit may also translate the commands to the N physical devices that are connected to the interface circuit via P independent data paths. The command translation may include, for example, routing the correct command directed to one of the M logical devices to the correct device (e.g. one of the N physical devices). Collectively, the P data paths connected to the N physical devices may optionally allow the interface circuit to guarantee that commands may be executed in parallel and independently, thus preventing command-scheduling constraints associated with the N physical devices. In this way the interface circuit may eliminate idle data-bus cycles or bubbles that would otherwise be present due to inter-device and intra-device command-scheduling constraints.

FIG. 9 illustrates a block diagram 900 showing a DDR3 SDRAM interface circuit disposed between an array of DRAM devices and a memory controller, in accordance with another embodiment. As an option, the block diagram 900 m ay be associated with the architecture and environment of FIGS. 1-8. Of course, the block diagram 900 may be associated with any desired environment. Further, the aforementioned definitions may equally apply to the description below.

As shown, a DDR3 SDRAM interface circuit 910 eliminates idle data-bus cycles due to inter-device and intra-device scheduling constraints. In the context of the present embodiment, the DDR3 SDRAM interface circuit 910 may include a command translation circuit of an interface circuit that connects multiple DDR3 SDRAM device with multiple independent data buses. For example, the DDR3 SDRAM interface circuit 910 may include command-and-control and address components capable of intercepting signals between the physical memory circuits and the system. Moreover, the command-and-control and address components may allow for burst merging, as described below with respect to FIG. 10.

FIG. 10 illustrates a block diagram 1000 showing a burst-merging interface circuit connected to multiple DRAM devices with multiple independent data buses, in accordance with still yet another embodiment. As an option, the block diagram 1000 may be associated with the architecture and environment of FIGS. 1-9. Of course, the block diagram 1000 may be associated with any desired environment. Further, the aforementioned definitions may equally apply to the description below.

A burst-merging interface circuit 1010 may include a data component of an interface circuit that connects multiple DRAM devices 1030 with multiple independent data buses 1040. In addition, the burst-merging interface circuit 1010 may merge multiple burst commands received within a time period. As shown, eight DRAM devices 1030 may be connected via eight independent data paths to the burst-merging interface circuit 1010. Further, the burst-merging interface circuit 1010 may utilize a single data path to the memory controller 820. It should be noted that while eight DRAM devices 1030 are shown herein, in other embodiments, 16, 24, 32, etc. devices may be connected to the eight independent data paths. In yet another embodiment, there may be two, four, eight, 16 or more independent data paths associated with the DRAM devices 1030.

The burst-merging interface circuit 1010 may provide a single electrical interface to the memory controller 1020, therefore eliminating inter-device constraints (e.g. rank-to-rank turnaround time, etc.). In one embodiment, the memory controller 1020 may be aware that it is indirectly controlling the DRAM devices 1030 through the burst-merging interface circuit 1010, and that no bus turnaround time is needed. In another embodiment, the burst-merging interface circuit 1010 may use the DRAM devices 1030 to emulate M logical devices. The burst-merging interface circuit 1010 may further translate row-activation commands and column-access commands to one of the DRAM devices 1030 in order to ensure that inter-device constraints (e.g. tRRD, tCCD, tFAW and tWTR etc.) are met by each individual DRAM device 1030, while allowing the burst-merging interface circuit 1010 to present itself as M logical devices that are free from inter-device constraints.

FIG. 11 illustrates a timing diagram 1100 showing continuous data transfer over multiple commands in a command sequence, in accordance with another embodiment. As an option, the timing diagram 1100 may be associated with the architecture and environment of FIGS. 1-10. Of course, the timing diagram 1100 may be associated with any desired environment. Further, the aforementioned definitions may equally apply to the description below.

As shown, inter-device and intra-device constraints are eliminated, such that the burst-merging interface circuit may permit continuous burst data transfers on the data bus, therefore increasing data bandwidth. For example, an interface circuit associated with the burst-merging interface circuit may present an industry-standard DRAM interface to a memory controller as one or more DRAM devices that are free of command-scheduling constraints. Further, the interface circuits may allow the DRAM devices to be emulated as being free from command-scheduling constraints without necessarily changing the electrical interface or the command set of the DRAM memory system. It should be noted that the interface circuits described herein may include any type of memory system (e.g. DDR2, DDR3, etc.).

FIG. 12 illustrates a block diagram 1200 showing a protocol translation and interface circuit connected to multiple DRAM devices with multiple independent data buses, in accordance with yet another embodiment. As an option, the block diagram 1200 may be associated with the architecture and environment of FIGS. 1-11. Of course, the block diagram 1200 may be associated with any desired environment. Further, the aforementioned definitions may equally apply to the description below.

As shown, a protocol translation and interface circuit 1210 may perform protocol translation and/or manipulation functions, and may also act as an interface circuit. For example, the protocol translation and interface circuit 1210 may be included within an interface circuit connecting a memory controller with multiple memory devices.

In one embodiment, the protocol translation and interface circuit 1210 may delay row-activation commands and/or column-access commands. The protocol translation and interface circuit 1210 may also transparently perform different kinds of address mapping schemes that depend on the expected arrival time of the column-access command. In one scheme, the column-access command may be sent by the memory controller at the normal time (i.e. late arrival, as compared to a scheme where the column-access command is early).

In a second scheme, the column-access command may be sent by the memory controller before the row-access command is required (i.e. early arrival) at the DRAM device interface. In DDR2 an DDR3 SDRAM memory systems, the early arriving column-access command may be referred to as the Posted-CAS command. Thus, part of a row may be activated as needed, therefore providing sub-row activation. In addition, lower power may also be provided.

It should be noted that the embodiments of the above-described schemes may not necessarily require additional pins or new commands to be sent by the memory controller to the protocol translation and interface circuit. In this way, a high bandwidth DRAM device may be provided.

As shown, the protocol translation and interface circuit 1210 may include eight DRAM devices to be connected thereto via eight independent data paths to. For example, the protocol translation and interface circuit 1210 may emulate a single 8 Gb DRAM device with eight 1 Gb DRAM devices. The memory controller may therefore expect to see eight banks, 32768 rows per bank, 4096 columns per row, and four bits per column. When the memory controller issues a row-activation command, it may expect that 4096 columns are ready for a column-access command that follows, whereas the 1 Gb devices may only have 2048 columns per row. Similarly, the same issue of differing row sizes may arise when 2 Gb devices are used to emulate a 16 Gb DRAM device or 4 Gb devices are used to emulate a 32 Gb device, etc.

To accommodate for the difference between the row sizes of the 1 Gb and 8 Gb DRAM devices, 2 Gb and 16 Gb DRAM devices, 4 Gb and 32 Gb DRAM devices, etc., the protocol translation and interface circuit 1210 may calculate and issue the appropriate number of row-activation commands to prepare for a subsequent column-access command that may access any portion of the larger row. The protocol translation and interface circuit 1210 may be configured with different behaviors, depending on the specific condition.

In one exemplary embodiment, the memory controller may not issue early column-access commands. The protocol translation and interface circuit 1210 may activate multiple, smaller rows to match the size of the larger row in the higher capacity logical DRAM device.

Furthermore, the protocol translation and interface circuit 1210 may present a single data path to the memory controller, as shown. Thus, the protocol translation and interface circuit 1210 may present itself as a single DRAM device with a single electrical interface to the memory controller. For example, if eight 1 Gb DRAM devices are used by the protocol translation and interface circuit 1210 to emulate a single, standard 8 Gb DRAM device, the memory controller may expect that the logical 8 Gb DRAM device will take over 300 ns to perform a refresh command. The protocol translation and interface circuit 1210 may also intelligently schedule the refresh commands. Thus, for example, the protocol translation and interface circuit 1210 may separately schedule refresh commands to the 1 Gb DRAM devices, with each refresh command taking 100 ns.

To this end, where multiple physical DRAM devices are used by the protocol translation and interface circuit 1210 to emulate a single larger DRAM device, the memory controller may expect that the logical device may take a relatively long period to perform a refresh command. The protocol translation and interface circuit 1210 may separately schedule refresh commands to each of the physical DRAM devices. Thus, the refresh of the larger logical DRAM device may take a relatively smaller period of time as compared with a refresh of a physical DRAM device of the same size. DDR3 memory systems may potentially require calibration sequences to ensure that the high speed data I/O circuits are periodically calibrated against thermal-variances induced timing drifts. The staggered refresh commands may also optionally guarantee I/O quiet time required to separately calibrate each of the independent physical DRAM devices.

Thus, in one embodiment, a protocol translation and interface circuit 1210 may allow for the staggering of refresh times of logical DRAM devices. DDR3 devices may optionally require different levels of zero quotient (ZQ) calibration sequences, and the calibration sequences may require guaranteed system quiet time, but may be power intensive, and may require that other I/O in the system are not also switching at the same time. Thus, refresh commands in a higher capacity logical DRAM device may be emulated by staggering refresh commands to different lower capacity physical DRAM devices. The staggering of the refresh commands may optionally provide a guaranteed I/O quiet time that may be required to separately calibrate each of the independent physical DRAM devices.

FIG. 13 illustrates a timing diagram 1300 showing the effect when a memory controller issues a column-access command late, in accordance with another embodiment. As an option, the timing diagram 1300 may be associated with the architecture and environment of FIGS. 1-12. Of course, the timing diagram 1300 may be associated with any desired environment. Further, the aforementioned definitions may equally apply to the description below.

As shown, in a memory system where the memory controller issues the column-access command without enough latency to cover both the DRAM device row-access latency and column-access latency, the interface circuit may send multiple row-access commands to multiple DRAM devices to guarantee that the subsequent column access will hit an open bank. In one exemplary embodiment, the physical device may have a 1 kilobyte (kb) row size and the logical device may have a 2 kb row size. In this case, the interface circuit may activate two 1 kb rows in two different physical devices (since two rows may not be activated in the same device within a span of tRRD). In another exemplary embodiment, the physical device may have a 1 kb row size and the logical device may have a 4 kb row size. In this case, four 1 kb rows may be opened to prepare for the arrival of a column-access command that may be targeted to any part of the 4 kb row.

In one embodiment, the memory controller may issue column-access commands early. The interface circuit may do this in any desired manner, including for example, using the additive latency property of DDR2 and DDR3 devices. The interface circuit may also activate one specific row in one specific DRAM device. This may allow sub-row activation for the higher capacity logical DRAM device.

FIG. 14 illustrates a timing diagram 1400 showing the effect when a memory controller issues a column-access command early, in accordance with still yet another embodiment. As an option, the timing diagram 1400 may be associated with the architecture and environment of FIGS. 1-13. Of course, the timing diagram 1400 may be associated with any desired environment. Further, the aforementioned definitions may equally appear to the description below.

In the context of the present embodiment, a memory controller may issue a column-access command early, i.e. before the row-activation command is to be issued to a DRAM device. Accordingly, an interface circuit may take a portion of the column address, combine it with the row address and form a sub-row address. To this end, the interface circuit may activate the row that is targeted by the column-access command. Just by way of example, if the physical device has a 1 kg row size and the logical device has a 2 kb row size, the early column-access command may allow the interface circuit to activate a single 1 kb row. The interface circuit can thus implement sub-row activation for a logical device with a larger row size than the physical devices without necessarily the use of additional pins or special commands.

FIG. 15 illustrates a representative hardware environment 1500, in accordance with one embodiment. As an option, the hardware environment 1500 may be implemented in the context of FIGS. 1-14. For example, the hardware environment 1500 may constitute an exemplary system.

In one exemplary embodiment, the hardware environment 1500 may include a computer system. As shown, the hardware environment 1500 includes at least one central processor 1501 which is connected to a communication bus 1502. The hardware environment 1500 also includes main memory 1504. The main memory 1504 may include, for example random access memory (RAM) and/or any other desired type of memory. Further, in various embodiments, the main memory 1504 may include memory circuits, interface circuits, etc.

The hardware environment 1500 also includes a graphics processor 1506 and a display 1508. The hardware environment 1500 may also include a secondary storage 1510. The secondary storage 1510 includes, for example, a hard disk drive and/or a removable storage drive, representing a floppy disk drive, a magnetic tape drive, a compact disk drive, etc. The removable storage drive reads from and/or writes to a removable storage unit in a well known manner.

Computer programs, or computer control logic algorithms, may be stored in the main memory 1504 and/or the secondary storage 1510. Such computer programs, when executed, enable the computer system 1500 to perform various functions. Memory 1504, storage 1510 and/or any other storage are possible examples of computer-readable media.

While various embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of a preferred embodiment should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents. 

What is claimed is:
 1. A sub-system, comprising: a first number of physical memory circuits comprising a first physical memory circuit, a second physical memory circuit, and a third physical memory circuit; an interface circuit electrically connected to the first physical memory circuit, the second physical memory circuit, and the third physical memory circuit via a first data bus, a second data bus, and a third data bus, respectively, wherein the interface circuit is configured to communicate with the first number of physical memory circuits and a system, wherein the interface circuit is configured to interface the first number of physical memory circuits that are associated with first command scheduling constraints, in order to simulate a different second number of virtual memory circuits that are associated with second command scheduling constraints, such that the first number of physical memory circuits appear to the system as the second number of virtual memory circuits that are associated with the second command scheduling constraints, wherein the second number of virtual memory circuits include a first virtual memory circuit and a second virtual memory circuit, wherein the first virtual memory circuit is simulated by the interface circuit using at least the first physical memory circuit, and the second virtual memory circuit is simulated by the interface circuit using at least the second physical memory circuit and the third physical memory circuit, wherein the interface circuit is configured to receive one or more system control signals directed to at least one of the second number of virtual memory circuits and to provide one or more physical control signals to at least one of the first number of physical memory circuits, and wherein the interface circuit is configured to: (i) access data from the first physical memory circuit via the first data bus and access data from the second physical memory circuit via the second bus, so as to access data from the first virtual memory circuit followed by the second virtual memory circuit without an inter-device command scheduling constraint, and (ii) access data from the second physical memory circuit via the second data bus and access data from the third physical memory circuit via the third bus, so as to access data from a first row of the second virtual memory circuit followed by a second row of the second virtual memory circuit without an intra-device command scheduling constraint.
 2. The sub-system as set forth in claim 1, wherein the first command scheduling constraints and the second command scheduling constraints include inter-device command scheduling constraints.
 3. The sub-system as set forth in claim 2, wherein the inter-device command scheduling constraints include at least one of a rank-to-rank data bus turnaround time or an on-die termination (ODT) control switching time.
 4. The sub-system as set forth in claim 1, wherein the first command scheduling constraints and the second command scheduling constraints include intra-device command scheduling constraints.
 5. The sub-system as set forth in claim 4, wherein the intra-device command scheduling constraints include at least one of a column-to-column delay time (tCCD), a row-to-row activation delay time (tRRD), a four-bank activation window time (tFAW), or a write-to-read turn-around time (tWTR).
 6. The sub-system as set forth in claim 1, wherein the interface circuit is further configured to issue, in place of the one or more system control signals directed to the at least one of the second number of virtual memory circuits, the one or more physical control signals directed to the at least one of the first number of physical memory circuits.
 7. The sub-system as set forth in claim 1, wherein the interface circuit is further configured to issue row-access commands and column-access commands.
 8. The sub-system as set forth in claim 1, wherein the one or more physical control signals are issued to different physical memory circuits utilizing separate busses.
 9. The sub-system as set forth in claim 1, wherein the interface circuit includes a circuit that is positioned on a dual in-line memory module (DIMM).
 10. The sub-system as set forth in claim 1, wherein the interface circuit is a buffer, a register, a memory controller, or an advanced memory buffer (AMB).
 11. The sub-system as set forth in claim 1, wherein the interface circuit and the first number of physical memory circuits take the form of a stack.
 12. The sub-system as set forth in claim 1, wherein the first number of physical memory circuits include a plurality of dynamic random access memory (DRAM) circuits.
 13. The sub-system in claim 1, wherein the one or more system control signals satisfy the second command scheduling constraints including an on-die termination (ODT) control switching time associated with the at least one of the second number of virtual memory circuits.
 14. The sub-system in claim 1, wherein the one or more physical control signals satisfy the first command scheduling constraints including an on-die termination (ODT) control switching time associated with the at least one the first number of physical memory circuits.
 15. A method, comprising: interfacing a first number of physical memory circuits that are associated with first command scheduling constraints, in order to simulate a different second number of virtual memory circuits that are associated with second command scheduling constraints, such that the first number of physical memory circuits appear to a system as the second number of virtual memory circuits that are associated with the second command scheduling constraints, wherein the second command scheduling constraints include fewer limitations for scheduling commands than the first command scheduling constraints, receiving one or more system control signals directed to at least one of the second number of virtual memory circuits; providing one or more physical control signals to at least one of the first number of physical memory circuits, wherein the first number of physical memory circuits include a first physical memory circuit, a second physical memory circuit, and a third physical memory circuit, wherein the interfacing further comprises electrically connecting to the first physical memory circuit, the second physical memory circuit, and the third physical memory circuit via a first data bus, a second data bus, and a third data bus, respectively, wherein the second number of virtual memory circuits include a first virtual memory circuit and a second virtual memory circuit, wherein the first virtual memory circuit is simulated using at least the first physical memory circuit, and the second virtual memory circuit is simulated using at least the second physical memory circuit and the third physical memory circuit; accessing data from the first physical memory circuit via the first data bus and accessing data from the second physical memory circuit via the second bus, so as to access data from the first virtual memory circuit followed by the second virtual memory circuit without an inter-device command scheduling constraint; and accessing data from the second physical memory circuit via the second data bus and accessing data from the third physical memory circuit via the third bus, so as to access data from a first row of the second virtual memory circuit followed by a second row of the second virtual memory circuit without an intra-device command scheduling constraint.
 16. A system, comprising: a host system; a first number of physical memory circuits comprising a first physical memory circuit, a second physical memory circuit, and a third physical memory circuit; and an interface circuit electrically connected to the first physical memory circuit, the second physical memory circuit, and the third physical memory circuit via a first data bus, a second data bus, and a third data bus, respectively, wherein the interface circuit is configured to communicate with the first number of physical memory circuits and the host system, the interface circuit configured to interface the first number of physical memory circuits in order to simulate a different second number of virtual memory circuits that are associated with second command scheduling constraints, such that the first number of physical memory circuits appear to the host system as the second number of virtual memory circuits that are associated with the second command scheduling constraints, wherein the second number of virtual memory circuits include a first virtual memory circuit and a second virtual memory circuit, wherein the first virtual memory circuit is simulated by the interface circuit using at least the first physical memory circuit, and the second virtual memory circuit is simulated by the interface circuit using at least the second physical memory circuit and the third physical memory circuit, and wherein the interface circuit is configured to receive one or more system control signals directed to at least one of the second number of virtual memory circuits and to provide one or more physical control signals to at least one of the first number of physical memory circuits, wherein the interface circuit is configured to: (i) access data from the first physical memory circuit via the first data bus and access data from the second physical memory circuit via the second bus, so as to access data from the first virtual memory circuit followed by the second virtual memory circuit without an inter-device command scheduling constraint, and (ii) access data from the second physical memory circuit via the second data bus and access data from the third physical memory circuit via the third bus, so as to access data from a first row of the second virtual memory circuit followed by a second row of the second virtual memory circuit without an intra-device command scheduling constraint.
 17. The system as set forth in claim 16, wherein the first number of physical memory circuits and the interface circuit are positioned on a dual in-line memory module (DIMM).
 18. The system as set forth in claim 16, wherein the first number of physical memory circuits and the interface circuit are positioned on a memory module that is configured to use a bus to communicate with a memory controller. 